(a) Field of the invention
The present invention relates to a decanter device for use to withdraw supernatant liquid from a tank containing a mixture of liquids of different densities or a mixture of a liquid and solid particles to be separated.
(b) Description of the Prior Art
Decanter devices of different structures are presently known and commonly used in the wastewater treatment industry to decant supernatant liquid from a tank or reservoir, usually called "reactor", containing a liquor to be processed, consisting of a mixture of liquids of different densities or a mixture of a liquid and solid particles.
The biological wastewater treatments which are carried out in the reactor, usually involve mechanical, biological, physical and/or chemical reactions and/or processings which result in an homogeneous mixing of the processed liquor. After sufficient reaction time, it is often necessary to separate the liquids and/or solids. Such a separation is normally done by allowing the mixture to sediment by gravity. To achieve such a sedimentation, mixing is simply interrupted for a period of time sufficient to allow for gravity separation to occur within the liquor. Then, the denser or heavier liquid or particles slowly deposit at the bottom of the reactor while the lighter or clearer liquid "moves" up to the surface, e.g. on top of the sediments, where it forms a supernatant liquid solution that can be decanted (e.g. removed) from the reactor through a pipe known as a "decanter device" leading out of the reactor and having at least one opening so located in the reactor as to enable withdrawing of the supernatant liquid.
As aforesaid, decanter devices of many kinds have been designed and are presently used to remove supernatant liquid solutions from reactors, especially from those used for carrying out the municipal and industrial wastewater treatment process known as "sequential batch reactor (SBR) treatment processes".
Considering that most applications, especially in SBR processes, require improved performances to cope with the continuous superior requirements for supernatant quality in terms of separation and suspended solids content, the decanter device to be used in the reactor tank must be very efficient and must satisfy several very specific requirements.
To better understand such requirements, it is worth reminding that during the reaction stage, the whole content of the tank is normally thoroughly mixed so that the liquor becomes homogeneous. Therefore, the solids are put in suspension and can enter the decanter device if the same simply consists of a hollow pipe or trough leading out of the tank.
During the settling stage, the solid particles that have already entered the decanter device settle within the device and thus become trapped inside. When the decanter device is operated to remove the supernatant liquid, the so-trapped solids are then flushed out of the tank, thereby greatly reducing the general performance of the process.
To avoid such a problem, three solutions have been proposed up to now.
The first solution consists in providing the decanter device with at least one gravity solids drainage opening. This opening is formed by inclined partitions merging together towards the bottom of the reactor. Such an opening can be used to withdraw the supernatant liquid as well as to drain the solids. The major inconveniences attributed to such a decanter are as follow.
First of all, keeping in mind that a high hydraulic capacity is a must to keep the productivity to its maximum, one can see that there is a real danger of disturbing the settled solids at the bottom of tank with such a decanter, where the supernatant liquid withdrawal orientation is toward from the bottom of the reactor. As a result, operation of such a device is limited, particularly if the decanter device is fixed and already submerged submerged at the desired low water level of the reactor.
Secondly, such a device does not prevent solids from penetrating. If solids are free to penetrate the decanter device, they must also be evacuated prior to operating the decanter. Some mechanical devices may succeed in doing a reasonable job but such mechanical devices must be checked regularly and maintained in proper order. A certain limitation of success is thus existing when using this type of device.
A second solution that has been proposed to solve the above mentioned problem consists in using a decanter device of the same type as above, and partially sealing the downward opening(s) of this decanter device, using an airlock to do so.
With such a design approach, a downward inclined partition opening is still used to withdraw the supernatant liquid and/or drain solids. The opening is however closed by an atmospheric air trap or lock that can be created by introduction of air in the pipe at the end of a decanting cycle or by a controlled vent line using a solenoid valve (see Canadian patent No. 1,249,228 to Mikkle Mandt).
The disadvantages of this second solution are as follow:
One again, it involves downward orientation of the supernatant liquid withdrawal.
Some solids can penetrate within of the air lock portion of the decanter and may be drained back prior to the decanting stage.
Air-trap seals are always subject to failure and may well suffer slow leaks. This may become critical particularly if not used frequently. Also, in the absence of proper heater/defrost system, ice formation in the vent line can be a serious source of problems.
The regulation opening must be in the form of a long narrow opening inside the unit. As proper head loss is critical to withdraw the liquid uniformly, a very narrow opening is used and represents a potential clogging/maintenance problem. The narrow opening is not accessible as it must be built inside the unit.
The unit must be built air tight and/watertight and thus is expensive to build. It is normally built in moulded fiberglass or stainless steel and its sealing integrity must be periodically re-checked.
A third solution that has been proposed so far to solve the above mentioned problem consists in designing the decanter device in such a manner that its inlet openings can be positioned completely out of the water so that it becomes impossible for any water/solids to penetrate during the reaction stage.
This solution is the most efficient and safe but requires serious mechanical involvements and thus quite complicated structure at the end.